1. Field of the Invention
The present invention relates in general to reducing distortion produced by nonlinear devices and more specifically to a method and apparatus for predistorting an input signal to a nonlinear device to reduce nonlinear distortion.
2. Description of the Related Art
Many electrical and electro-optic devices have nonlinear characteristics that result in undesired distortion in an output signal. Optical communication systems are particularly susceptible to nonlinear distortion resulting in reduced performance. Communication systems use transmitters having signal modulators that modulate a desired signal onto a carrier signal before transmission. Optical communication systems commonly employ high power optical transmitters. For example, cable television broadcast networks use high power optical transmitters to transport multiple video channels through fiber optics for signal distribution. These transmitters typically utilize electro-optic intensity modulators to modulate an optical signal before transmission.
Electro-optic modulators are constructed by placing metal electrodes above and below an optical waveguide formed in a crystal. Lithium niobate (LiNbO3) is most often used for the crystal although other materials may be used such as lithium tantalate (LiTaO3). A desired signal is modulated onto these electrodes, and the electric field between the electrodes, in turn, modulates the crystal, changing its optical index of refraction in response to the input signal. As the refractive index changes, the light (typically from a continuous wave (CW) source laser) is modulated as it passes through the external modulator.
Electro-optic modulators have a sinusoidal transfer function of modulation voltage input (across the electrodes) in relation to light modulation output. Consequently, electro-optic modulators are nonlinear devices that generate undesired harmonics when a modulating signal is injected into the modulator.
The non-linear characteristic of electro-optic modulators is a problem in multichannel optical communication systems such as cable television (CATV) systems. In CATV systems, a large number of closely spaced carriers require linear operation of the system in order minimize intermodulation distortion.
One method of minimizing the harmonics includes operating the modulator symmetrically around a quadrature bias point. By injecting low level signals, the modulator can be operated in the near linear portion of the transfer function curve. As the amplitude of the modulation signal is increased, however, the harmonics due to the non-linear characteristics of the modulator increase significantly. As a result, the dynamic range of the modulator is significantly limited.
Other methods of reducing harmonics include xe2x80x9cpredistortingxe2x80x9d the input signal prior to injection into the electro-optic modulator. These predistortion methods attempt to approximate an arcsine function that predistorts the RF input signal.
One such predistortion method, described in U.S. Pat. No. 5,168,534, discloses a linearization circuit that uses an electro-optic modulator as part of a predistortion circuit approximating the arcsine function. This attempt, however, requires that the phase and gain accuracy be tightly constrained. Further, this attempt is costly because two modulators are required.
Another predistortion method, described in U.S. Pat. No. 5,327,279, attempts to approximate the arcsine function using a diode pair to approximate a sine function and combining twice the input signal in an attempt to generate a 2(x)-sin(x) function. The diode pair only approximates a sine function and performance is limited.
Therefore, there exists a need for a method and apparatus for reducing the distortion produced by a non-linear device.
The present invention relates to a method and apparatus for reducing distortion due to characteristics of a nonlinear device.
In an embodiment of the present invention, a predistortion circuit includes a signal splitter, a diode network and a signal combiner. An input signal is split into two portions in the signal splitter. One of the signal portions is passed through a diode network and combined with the other portion of the input signal in the signal combiner. The diode network includes a plurality of diode circuits, each including two diode branches coupled to an electrical ground. The resulting predistorted signal at the output of the signal combiner is amplified, amplitude adjusted and injected into the nonlinear device. The input signal is predistorted such that distortion due to nonlinear characteristics of the nonlinear device is reduced.